We used to sell a couple of brands of chronometers in the gun shop where I used to work.
The "cheap" one, had 2 "gates" a foot apart, as the round passed through it broke a strip of foil.
Only problem with that one was with a REALLY high speed round, 220 Swift, 22-250 or 17, the projectile would often disintegrate striking one or the other foil strip.
That unit was AUS$800, back in the '80's.
The other unit used 2 light gates a foot apart.
It was a little better, but not by much.
It cost AUS$1800!!!!
I have seen "modern" units that use 2 magnetic gates that I'll assume exploit eddy currents as the spinning projectile goes through them.
Back in ye olde times, they used to work out impact velocity by hitting a known mass of lead and measuring how far it moved, either with a pendulum or the lead block simply got pushed back.
You need to decide if you want to measure muzzle velocity or impact velocity.
In the back section of the RCBS reloading guide there is a whole section devoted to calculating bullet drop, impact velocity and windage.
These equations all take into account projectile profile, muzzle velocity, air temperature, humidity, barometric pressure and firing angle.
For a given projectile weight the profile be it, spire point, round nosed, hollow point, boat tail or combinations of each will make a HUGE difference.
My favourite 303 British loads, 180gr round nosed and 165gr spire points have around the same CEP.
But only because I spent ages at the range experimenting with different powders and loads.
I mainly did that so I could use both rounds with the same sight settings.
Now that I have digital accelerometers I've been wondering if I could mount one on my rifle a measure how much recoil acceleration there is to calculate muzzle velocity.
You'd have to bench fire, which is no big deal, but it would give you a ball park figure.